Learn / The basics
Sudoku basics · lesson 1 of 8 · 3 minutes
How to play Sudoku
Sudoku is a logic puzzle, not a math puzzle. Here are the rules in plain English, and how to make your first move.
The whole goal
Fill every empty square with one digit from 1 through 9. Every horizontal row, every vertical column, and every outlined 3 by 3 box must contain each digit exactly once. The digits are labels: you never add, subtract, or compare their size.
For exampleA row containing 1, 2, 3, 4, 5, 6, 7, 8 and one blank is missing 9.
A cell belongs to three groups
Choose one empty cell. Look across its row, down its column, and inside its box. Its answer must work in all three groups at the same time. A digit already filled anywhere in one of those groups is ruled out for your cell.
For exampleIf the row has a 4, your cell cannot be 4, even when its column and box have no 4 yet.
Clues stay put
The darker original digits are clues supplied with the puzzle. They are fixed. The other large digits are your entries and can be erased or undone. Small pencil marks mean possible answers, not placed answers. A puzzle is finished only when every cell is filled and all three rules hold.
For exampleTwo small 7s in the same row are allowed. Two large, placed 7s in that row are a conflict.
Make one certain move
Start with a row, column, or box that has just one empty cell. List the digits already there, find the missing digit, then check that cell’s other two groups. Place it and look again: one answer often makes the next move easier.
For exampleYou do not need to see the whole solution. One justified placement is enough progress.
Quick check
A blank cell has no 6 in its row or column, but its box already has a 6. Can this cell be 6?
- Yes, because two of its groups allow it.
- No, because the box already contains 6.
- Only if there are no other empty cells.
Show the answer
No, because the box already contains 6. Exactly. Rows, columns, and boxes apply together. The existing 6 rules it out.
Next, when you are ready: Full house.